Despite the revolutions represented by GPS technology and advanced direction-of-arrival algorithms (DoA), the problem of determining precise (<1 m resolution) positions of wildlife radio transmitters remains to date essentially unsolved. There are a number of reasons for this, principal among them being the low-duty cycle nature of the signals (required by practical energy/payload constraints), which prevents coherent demodulation. Payload constraints (<1 gram for small species of animals) also preclude the inclusion of ultra-precise clocks into transmitters which in turn prevents the implementation of sub-meter precision time-of-arrival location methods.
Much of the existing work on radio location has been tailored to the CDMA based cellular telephony and GPS applications. Both these applications have the benefits of relatively high transmission power, coherent demodulation and stablity. Cellular telephony localization schemes are typically based on statistical measurement of signal strength, direction of arrival (DoA), time of arrival (ToA) or time difference of arrival (TDoA) as a means to position a radio source: Caffrey, J. J., Wireless Location in CDMA Cellular Radio Systems, (Kluwer academic Publishers: Norwell, Mass., 2000).
Although DoA methods such as beam forming can be applied to wildlife radio transmitter location finding, estimator precision will vary according to the number of antenna elements used and the relative location of the radio source from the antenna array. Moreover, unambiguous location finding by means of beamforming necessitates specific antenna geometries that could under field conditions restrict the applicability of the technique.
DoA methods based on orthogonal projection such as MUSIC (Schmidt, “Multiple Emitter Location and Signal Parameter Estimation,” IEEE Transactions on Antennas and Propagation, vol. AP-34, No. 3, (March 1986), pp. 276-280,) offer improved precision performance over classic beamforming. Such methods present two drawbacks for wildlife monitoring. First, the number of radio sources that can be monitored cannot exceed the number of antennas. Typically, wildlife studies involve the simultaneous monitoring of large numbers of transmitter tagged animals. Second, an appropriate array manifold selection must be made (Schmidt, “Multilinear Array Manifold Interpolation”, IEEE Transactions on Signal Processing, vol. 40, No. 4, (April 1992), pp. 857-866). This typically entails an empirical measurement of the antenna manifold. That is to say that a radio transmitter must be used to calibrate the system by moving the device across a fine grid of points encompassing the search area. In most animal monitoring applications the empirical calibration of the array manifold would be logistically or economically impractical.
Location finding based on ToA and TDoA techniques (including GPS) are geometry dependent (i.e. transmitter and receiver locations determine precision levels) and are precluded from use in small animal applications due to the inability to perform coherent demodulation and the current technological limitation of incorporating an ultra-stable clock source on-board a miniature (<1 gram) transmitter.
The measurement-based location-finding approach in Wax et al. in U.S. Pat. No. 6,104,344 (which is not admitted to be prior art with respect to the present invention by its mention in this Background section) provides an alternative to past radiolocation finding methods. The approach calls for a library of covariance matrix related signatures to be empirically collected across the search array. This library is then compared using a statistical technique against measured signatures from mobile transmitters to determine location. This approach, like the empirical DoA manifold method outlined by Schmidt, has limited applicability to wildlife tracking from a logistic, portability and economic perspective.